Spray a baking dish (8x8 or 9x9 works well) with olive oil cooking spray. Add the Brussels sprouts that you have cut in half to the baking dish. Spray with more cooking spray and season with salt and pepper to taste.
Bake the Brussels sprouts for 15 minutes. Then remove from the oven, stir the veggies and roast for another 10 minutes.
While the sprouts roast the final 10 minutes, start making the sauce. Begin by melting ½ tablespoon of butter over medium heat. Then add the shallots and cook for about 4 minutes or until they are softened.
Add 2 teaspoons of flour to the shallots and mix for 30-60 seconds to coat the shallots and cook the flour. Then pour ¾ cup of milk and stir constantly for about 4-5 minutes or until the sauce is bubbling and has thickened. Finally add 1 ounce of Gruyere cheese and 1 tablespoon of grated Parmesan cheese along with the thyme, and combine. The sauce is ready once the cheese has melted.
When the Brussels sprouts are done roasting, spoon the cheese sauce over the top and then sprinkle the remaining ounce of Gruyere cheese over the top and bake for 15 more minutes or until the sauce is bubbly and the cheese is turning a light brown. Serve right away and enjoy!
Recipe by Can Cook, Will Travel at https://cancookwilltravel.com/skinny-brussels-sprouts-gratin/